1. Field of the Invention
The invention relates to a communication system which is provided with a first device and a second device that are connected by a single communication line, and which performs two-way simultaneous communication between the two devices.
2. Description of the Related Art
Japanese Patent Application Publication No. JP-A-8-265308, for example, describes a communication system that performs two-way simultaneous communication between two stations using a single communication line. The communication system described in that publication transmits data from a first station to a second station using a pulse signal set at a duty ratio that differs according to the data, and transmits data from the second station to the first station using a pulse signal having an amplitude value that differs according to the data. At the first station, the data transmitted from the second station is classified based on the amplitude value of the pulse signal transmitted over the communication line, while at the second station, the data transmitted from the first station is classified based on the duty ratio of the pulse signal transmitted over the communication line. As a result, even if data is transmitted from the two stations simultaneously over the single communication line, all of the data is able to be read without interference.
In the foregoing communication system, however, the pulse signal transmitted from the first station changes the duty ratio according to the data and its cycle is always fixed. Therefore, the data transmission speed from the first station to the second station ends up being fixed. If that communication method is employed for data communication in a system in a vehicle provided with vehicle state control units for controlling a plurality of vehicle states, in which those control units operate by transmitting and receiving data to and from each other, for example, the data transmission speed when an abnormality is detected ends up being the same as the data transmission speed during normal operation, which is undesirable. That is, when an abnormality is detected, an abnormality processing command is preferably transmitted to the appropriate control unit quicker than data is transmitted during normal operation. This, however, is not possible with the foregoing communication method.